Spindle pole body-anchored Kar3 drives the nucleus along microtubules from another nucleus in preparation for nuclear fusion during yeast karyogamy

BIK1 function is required for nuclear fusion, chromosome disjunction, and nuclear segregation during mitosis. The BIK1 protein colocalizes with tubulin to the spindle pole body and mitotic spindle. Synthetic lethality observed in double mutant strains containing a mutation in the BIK1 gene and in the gene for alpha- or beta-tubulin is consistent with a physical interaction between BIK1 and tubulin. Furthermore, over- or underexpression of BIK1 causes aberrant microtubule assembly and function, bik1 null mutants are viable but contain very short or undetectable cytoplasmic microtubules. Spindle formation often occurs strictly within the mother cell, probably accounting for the many multinucleate and anucleate bik1 cells. Elevated levels of chromosome loss in bik1 cells are indicative of defective spindle function. Nuclear fusion is blocked in bik1 x bik1 zygotes, which have truncated cytoplasmic microtubules. Cells overexpressing BIK1 initially have abnormally short or nonexistent spindle microtubules and long cytoplasmic microtubules. Subsequently, cells lose all microtubule structures, coincident with the arrest of division. Based on these results, we propose that BIK1 is required stoichiometrically for the formation or stabilization of microtubules during mitosis and for spindle pole body fusion during conjugation.

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Spindle pole body fusion in the smut fungus Tilletia foetida

Canadian Journal of Botany ◽

10.1139/b86-166 ◽

1986 ◽

Vol 64 (6) ◽

pp. 1221-1223 ◽

Cited By ~ 4

Author(s):

Blair J. Goates ◽

James A. Hoffmann

Keyword(s):

Nuclear Fusion ◽

Spindle Pole Body ◽

Spindle Pole ◽

Smut Fungus ◽

Narrow Region ◽

Pole Body ◽

Spindle Pole Bodies ◽

Tilletia Foetida

Fusion of double-structured, interphase spindle pole bodies (SPBs) occurred before nuclear fusion in heterokaryotic secondary sporidia. The SPBs of two separate nuclei were juxtaposed with their long axes perpendicular to each other. Also, SPBs were observed oriented with their long axes parallel and fused to each other at both ends. Fusion apparently continued toward the midportion of the SPBs. Nuclei were observed joined together in a narrow region. These nuclei appeared to share a single SPB that was located opposite to a protuberance on both nuclei. Following fusion, the SPB apparently returned to an interphase structure.

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KAR5 Encodes a Novel Pheromone-inducible Protein Required for Homotypic Nuclear Fusion

The Journal of Cell Biology ◽

10.1083/jcb.139.5.1063 ◽

1997 ◽

Vol 139 (5) ◽

pp. 1063-1076 ◽

Cited By ~ 25

Author(s):

Christopher T. Beh ◽

Valeria Brizzio ◽

Mark D. Rose

Keyword(s):

Membrane Fusion ◽

Nuclear Membrane ◽

Nuclear Fusion ◽

Spindle Pole Body ◽

Spindle Pole ◽

Deletion Mutations ◽

Pheromone Response Pathway ◽

Pole Body ◽

Inducible Protein ◽

Novel Protein

KAR5 is required for membrane fusion during karyogamy, the process of nuclear fusion during yeast mating. To investigate the molecular mechanism of nuclear fusion, we cloned and characterized the KAR5 gene and its product. KAR5 is a nonessential gene, and deletion mutations produce a bilateral defect in the homotypic fusion of yeast nuclei. KAR5 encodes a novel protein that shares similarity with a protein in Schizosaccharomyces pombe that may play a similar role in nuclear fusion. Kar5p is induced as part of the pheromone response pathway, suggesting that this protein uniquely plays a specific role during mating in nuclear membrane fusion. Kar5p is a membrane protein with its soluble domain entirely contained within the lumen of the endoplasmic reticulum. In pheromone-treated cells, Kar5p was localized to the vicinity of the spindle pole body, the initial site of fusion between haploid nuclei during karyogamy. We propose that Kar5p is required for the completion of nuclear membrane fusion and may play a role in the organization of the membrane fusion complex.

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kem mutations affect nuclear fusion in Saccharomyces cerevisiae.

Genetics ◽

10.1093/genetics/126.4.799 ◽

1990 ◽

Vol 126 (4) ◽

pp. 799-812 ◽

Cited By ~ 5

Author(s):

J Kim ◽

P O Ljungdahl ◽

G R Fink

Keyword(s):

Saccharomyces Cerevisiae ◽

Null Allele ◽

Nitrogen Starvation ◽

Nuclear Fusion ◽

Spindle Pole Body ◽

Spindle Pole ◽

Wild Type ◽

Reading Frame ◽

Pole Body ◽

Fusion Defect

Abstract We have identified mutations in three genes of Saccharomyces cerevisiae, KEM1, KEM2 and KEM3, that enhance the nuclear fusion defect of kar1-1 yeast during conjugation. The KEM1 and KEM3 genes are located on the left arm of chromosome VII. Kem mutations reduce nuclear fusion whether the kem and the kar1-1 mutations are in the same or in different parents (i.e., in both kem kar1-1 X wild-type and kem X kar 1-1 crosses). kem 1 X kem 1 crosses show a defect in nuclear fusion, but kem 1 X wild-type crosses do not. Mutant kem 1 strains are hypersensitive to benomyl, lose chromosomes at a rate 10-20-fold higher than KEM+ strains, and lose viability upon nitrogen starvation. In addition, kem 1/kem 1 diploids are unable to sporulate. Cells containing a kem 1 null allele grow very poorly, have an elongated rod-shape and are defective in spindle pole body duplication and/or separation. The KEM 1 gene, which is expressed as a 5.5-kb mRNA transcript, contains a 4.6-kb open reading frame encoding a 175-kD protein.

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